1. Technical Field
The invention relates to the field of loop free network topology domains. In particular, the invention relates to inter-domain loop protection using a hierarchy of loop resolving protocols.
2. Background Information and Description of Related Art
A virtual local area network (VLAN) restricts communication between computers in a network on some network administrator defined basis rather than, for example, the physical location of the computers. The computers in the VLAN communicate with each other as if they are connected to the same local area network (LAN) segment even though they may actually be physically located on different segments of an internetwork. Conversely, computers connected to the same network segment may not necessarily be able to communicate with each other if, for example, they belong to different VLANs.
Networks, such as LANs, are often partitioned into separate segments to increase network performance. Bridges or switches connect the segments. A LAN may include redundant bridges and paths to provide backup in the event of a bridge or path failure in the LAN. The Spanning Tree Protocol (STP), part of the Institute of Electrical and Electronics Engineers (IEEE) 802.1D standard, authored and published by the IEEE Standards Association in June 1998, comprises an algorithm that provides for this redundancy while preventing undesirable loops in the LAN. Using the algorithm, bridges select a root bridge, and calculate and select a single path to the root bridge when faced with multiple redundant paths. After path costs are calculated and a single path is selected, the bridges block the redundant links.
There are other algorithms that prevents undesirable loops in a network. For example, the Ethernet Automatic Protection Switching Protocol (EAPS) prevents loops in a Layer-2 (L2) network having a ring topology. In the EAPS system, there is a master node linked to at least one transit node in a ring network. Each node has a primary port and a secondary port. During normal operation, the master node blocks data traffic from traversing its secondary port. During a network failure, the master node reroutes data traffic through its secondary port. Thus, similar to STP, EAPS blocks ports to prevent loops and provide a loop free domain.
These algorithms have been modified in different ways to further accommodate VLANs. For example, the IEEE 802.1Q standard, authored and published by the IEEE Standards Association in March 1999, specifies a single STP instance for all VLANs while another approach provides for a separate STP instance for each VLAN. Yet another approach contemplates one or more STP instances for one or more VLANs. Thus, an STP instance or STP domain (STPD) may span multiple VLANs, or a single VLAN may encompass multiple STPDs.
Similarly, other algorithms, such as EAPS provide for a domain to span multiple VLANs, or a single VLAN to encompass multiple loop free domains. However, one problem with multiple loop free domains is the possibility of inter-domain loops through the switches that are members of more than one loop free domain, since algorithms, such as STP and EAPS, only guarantees a loop-free topology inside each domain. Furthermore, a network, such as a VLAN, may contain different types of loop free domains with the possibility of inter-domain loops through switches that are members of more than one type of loop free domain.